The invention relates to digital audio signal processing. More particular the invention relates to audio signal quantization.
In very-low-bit-rate transform coding, the number of bits per frame are generally not sufficient to avoid artifacts in the decoded signal. Musical noise, in particular, can appear in stationary music or noise spectra due to transform lines (bins) being “turned on and off”, i.e. quantized to zero or not quantized to zero, at a certain frequency from one frame to the next. Not only does such a coding approach give the decoded signal region a more tonal character than the original signal has (hence the term musical noise), it also does not yield a notable advantage over not coding said spectral region at all and instead applying a bin-replacement technique like the noise filling algorithms in the TCX or FD coding systems employed in xHE-AAC [4]. In fact, the explicit but insufficient coding of regions prone to musical coding noise necessitates bits in the entropy coding stage of the transform coder, which sonically are better spent in other spectral regions, especially at low frequencies where the human auditory system is sensitive.
One way of reducing the occurrence of musical noise in low-bit-rate audio coding is to modify the behavior of the quantizer mapping the input spectral lines to quantization indices so that it adapts to the instantaneous input signal characteristic and bit consumption of the quantized spectrum. More precisely, a dead-zone used during quantization is altered signal-adaptively. Several approaches have been published [5, 6, and references therein]. In [5], the quantizer adaptation is performed on the entire spectrum to be coded. The adapted quantizer therefore behaves identically for all spectral bins of the given frame. Moreover, in case of quantization with the optimal dead-zone zopt, 2 bits of side-information has to be transmitted to the decoder, representing a bit-rate and backward-compatibility penalty. In [6], the quantizer is adapted on a per-frequency-band basis, but two quantization attempts are conducted per band, and only the better attempt (according to a certain decision) is used for transmission. This is complex.